Software-defined networking provides a promising approach to build and operate high-performance networks with high efficiency and affordable cost. While SDN is of great importance to ISPs, it is infeasible to replace the entire existing infrastructure with a clean slate of SDN design. Most early SDN practices concentrate on small-scale greenfield designs. Here, we focus on mission-critical large-scale backbone networks, and discuss the key technologies and applications for ISPs' SDN adoption. Taking an evolutionary viewpoint, we discuss strategies and approaches to integrate SDN capabilities with traditional technologies to achieve the high-performance traffic engineering, fast service provisioning, and rapid failure restoration required for existing network operations.